1. Field of the Invention
The present invention relates to a defective pixel data correcting apparatus, an image capturing apparatus, and a method for correcting defective pixel data, and more particularly relates to a technique for correcting a defective pixel of an image sensor.
2. Description of the Related Art
Digital cameras and digital video cameras use image sensors typified by CMOS image sensors and CCD image sensors. While the image sensors each have hundreds of thousands to tens of millions of pixels arranged, the image sensors include a pixel which fails to generate a signal to be output from the pixel (hereinafter, referred to as a normal output signal), due to crystal faults of a material forming the image sensor and dust. Such a pixel is referred to as a defective pixel, and an abnormal signal output by the defective pixel is referred to as an abnormal output signal in the specification. It is to be noted that the output of an abnormal output signal may include a state in which no signal is output at all.
Defective pixels are classified broadly into two main categories: steady defective pixels which output an abnormal output signal in a steady manner; and nonsteady defective pixels which output a normal output signal and an abnormal output signal in an unsteady manner.
It is known that the output characteristics for abnormal signals differ between steady defective pixels and nonsteady defective pixels in CMOS image sensors. In the case of excluding sensitivity dependent defective pixels such as dust and aperture unevenness, white defects due to crystal faults of a light receiving unit make up a majority of steady defective pixels. Therefore, a white defective pixel is defined as a steady defective pixel. Since the white defective pixel is accompanied by an increase in dark signal, the abnormal signal output level has characteristics that depend on the temperature of the defective pixel and the charge accumulation period of the defective pixel.
On the other hand, nonsteady defective pixels differ from steady defective pixels in points where crystal faults are caused, and the characteristics of the abnormal output signal level for the nonsteady defective pixels thus have almost no dependence on the temperature or charge accumulation period of the defective pixel.
In a general circuit configuration for a pixel of a CMOS image sensor, shown in FIG. 6, reference numerals 801 and 802 respectively denote a photodiode (PD) and a reset CMOS for resetting accumulated electric charges. In addition, reference numerals 803 and 804 respectively denote a floating diffusion (FD) for detecting electric charges and a pixel source follower.
Then, many steady defective pixels are caused by crystal faults of the PD 801, whereas nonsteady defective pixels are considered to be caused by repeating electron capture and electron emission at the interface state of a MOS transistor constituting the pixel source follower 804. The abnormal output signal levels for the nonsteady defective pixels have almost no dependence on the temperature or the charge accumulation period because the pixel source follower 804 causes the abnormality.
Conventionally, a technique has been proposed for appropriately correcting nonsteady defective pixels and steady defective pixels by changing the correction method depending on the type of defective pixels (Japanese Patent Laid-Open No. 2008-131273).
However, Japanese Patent Laid-Open No. 2008-131273 fails to take into particular consideration the fact that the dependence of the abnormal signal output level on the pixel temperature and charge accumulation period differs between nonsteady defective pixels and steady defective pixels. Therefore, in a case in which the pixel temperature or charge accumulation period differs between during defective pixel detection and during subject image capturing, there is a possibility that the correction to, in particular, nonsteady defective pixels is not appropriately carried out due to the difference in dependence as described above. For example, in a case in which a defective pixel to be corrected is to be selected while a steady defective pixel highly dependent on the temperature and charge accumulation period is used as a standard, a nonsteady defective pixel to be corrected is not selected or a nonsteady defective pixel not to be corrected is selected. In the former case, the correction has not been completed, thereby resulting in insufficient correction to noise of the image. In the latter case, unnecessary correction is carried out, thereby resulting in the problem of degradation in resolution.